Recently, immersion type membrane filtration methods have been utilized in a variety of fields, the methods involving immersing a membrane module in a fluid to be processed and filtrating the fluid to be processed that is caused to be naturally circulating by taking advantage of the buoyancy of air bubbles (for example, refer to Japanese Patent Laid-Open No. 61-129094). The method is an energy conservation precision process for filtrating highly contaminated liquids such as an activated sludge processed fluid in place of an ultrafiltration method that involves forcibly supplying a fluid to be processed to a membrane module by mechanical circulating means such as a pump and circulating.
Membrane modules for use in such immersion type membrane filtration methods are generally called immersion type filtration membrane modules and are exclusively a hollow fiber module and a flat sheet module (for example, see “Report on Commercialization of Small-Size Domestic Wastewater Processing Apparatus that Introduces a Membrane Processing Method, 1992 to 1995 Edition,” Japan Education Center of Environmental Sanitation). Here, a hollow fiber module is fabricated by collecting numerous hollow fiber membranes with both outside and inside diameters of a few millimeters and fixing both ends. The module can filtrate a fluid to be processed by reducing the inside pressure of the hollow fiber membrane and passing the fluid to be processed from the outside into the inside thereof. On the other hand, a flat sheet module comprises a plurality of flat sheet membranes set like a bag. It can filtrate a fluid to be processed by reducing the inside pressure of the flat sheet membrane and passing the fluid to be processed from the outside into the inside thereof. Incidentally, in a hollow fiber module, why a fluid to be processed is rendered to pass from the outside to the inside of the hollow fiber membrane is that, because of the inside diameter of the hollow fiber membrane being almost the same as the size of materials to be filtrated and activated sludge, contained in a fluid to be processed, passage of the fluid to be processed from the inside to the outside of the hollow fiber membrane raises the possibility of the hollow fiber membrane being clogged and closed, and that air bubbles hardly pass through the inside of the hollow fiber membrane leading to the difficulty of natural circulation of the fluid to be processed. On the other hand, in a flat sheet module, passage of a fluid to be processed from the outside to the inside of the flat sheet membrane is likely to form, due to air bubbles, an upward-moving stream of the fluid to be processed required for rendering the fluid to be processed to naturally circulate.
When comparing the hollow fiber membrane with the flat sheet module, the hollow fiber module has a larger membrane area per volume than the flat sheet module, and thus can enhance the volume efficiency. In other words, for a hollow fiber module and a flat sheet module with the same membrane area, the hollow fiber module can be made more compactly. In contrast, a flat sheet module is large in flux (filtration flow rate per unit area of membrane) as compared with a hollow fiber module, and therefore it can perform immersion type membrane filtration more efficiently than the hollow fiber module. Therefore, an immersion type membrane filter apparatus utilizing these immersion type filtration membrane modules is substantially difficult to conduct miniaturization as well as enhancing the filtration efficiency at the same time.
In addition, in these immersion type filtration membrane modules, the flux is increased with increasing air bubble amount to be supplied, and so the energy cost for supplying air bubbles is increased in order to enhance the filtration efficiency by attaining a specified amount of flux. Also, in the immersion type membrane filter apparatus using a hollow fiber module, the increase of the volume of air bubbles to be supplied requires ease of the air bubbles being passed by setting large the clearance between hollow fiber membranes, and thus it becomes difficult to enhance the volume efficiency of the hollow fiber module.
The object of the present invention is to realize an immersion type membrane filter apparatus being a small size as well as being capable of economically enhancing the filtration efficiency.